1. Field of the Invention
This invention relates to a memory socket, and particularly, it relates to a memory socket which is used to mount a memory card, as a detachable component, on a printed board, and the length of wiring.
2. Description of the Related Art
Recently, the minatureization and enhancement of the performance of a computer or a network device, etc., have been rapidly achieved. To this end, large-scale integration of components, an increase in processing speed, and a high-density mounting technology for mounting the components on the printed circuit board have been required at each stage of development of products.
FIG. 1 shows an example of a router device, and FIG. 2 shows an example of an arrangement of components on a daughter board shown in FIG. 1.
As shown in FIG. 1, the router device 10 is comprised of a power supply unit 11, a back panel 12 which supplies electricity to a plurality of circuit boards mounted thereon from the power supply unit 11 and connects signal lines among the printed circuit boards, a motherboard 13 and a daughter board 14, which is an extension board for the motherboard 14, mounted on the back panel 12.
The daughter board 14, shown in FIG. 2, is provided with connectors 15 and 16 for connecting power supply lines and the signal lines, etc. to those of the motherboard 13 opposed to the daughter board 14. A CPU chip 17 provides a high-speed clock signal of 100 MHz to memory cards (DIMM) 18 and 19 comprised of synchronous type DRAMs, such as SDRAMs (Synchronous DRAMs), to carry out clocked synchronization of the memory cards in order to operate them logically as a single memory.
Each of the memory cards 18 and 19 is horizontally mounted on the daughter board 14 because of the distance (see FIG. 1) between the motherboard 13 and the daughter board 14. In the example, the CPU chip 17 of a clock signal source is located closer to the memory card 18 than the memory card 19. Therefore, a bypass line is provided to a line connecting between the memory card 18 and the CPU chip 17, so that no time difference (skew) occurs between the two memory cards 18 and 19.
Due to the bypass route, the wiring length between the memory card 18 and the CPU chip 17 is identical to that between the memory card 19 and the CPU chip 17 (this arrangement rule is referred to as “equal-length wiring”). In the example, the clock period of 100 MHz clock signal is 10 ns and the clock width thereof is one-half of the clock period, i.e., 5 ns.
The loss of a design margin of a few ns is caused due to various conditions including skew, the wiring capacity, irregularity of the components, the ambient temperature, the temperature of the chip during operation, etc., which result in a lack of a guarantee of correct operation. To prevent this, the designing of the wiring is conducted in such a way that the wiring length is made as short as possible and that equal-length wiring is established.
FIG. 3 shows an example, similar to an example shown in FIG. 2, in which conventional memory sockets are arranged in parallel and are oriented in the same direction.
The equal-length wiring can be easily obtained because, in this example, the clock pins of the memory cards 18 and 19 can be placed at an equal distance from the clock source 17.
However, it is necessary to provided a space for inserting and removing operations of the memory cards therebetween. Therefore, a group of memory slots unnecessarily occupies an area of the board. This makes it difficult to reduce the size and cost of the device.
FIG. 4 shows an example in which conventional memory sockets are arranged in parallel and are oriented in opposite directions.
In this example, in order to reduce an area of a group of the memory sockets shown in FIG. 3, a memory socket of the memory card 19 is turned in a horizontal plane by 180 degrees to face in a direction opposite to that shown in FIG. 3 and is attached to the board. Thus, the space between the memory cards 18 and 19 for inserting and removing operations of the memory card can be made smaller.
However, the opposed pins are arranged in opposite orders and a bypass route necessary to realize the equal-length wiring must be provided on one of the memory sockets. Consequently, wiring design is made difficult and the area of the bypass route is increased. As mentioned above, the examples shown in FIG. 3 and FIG. 4 have advantages but also have problems to be solved.